1. Technical Field of the Invention
The present invention relates to locating the geographic position of a mobile station operating within a cellular telephone network and, in particular, to proactively providing geographic position information following inter-exchange handoff.
2. Description of Related Art
Conventional cellular telephone technology, by itself, does not include the capability of pinpointing, with any reasonable or useful degree of accuracy, the location of the mobile subscriber. For example, using a conventional cellular telephone network, the extent of the position information precision typically available is to identify the cell within which a mobile station is located. Some cells, though, have a coverage radius in excess of one kilometer. Thus, cell location identification accordingly provides little, if any, assistance in actually locating the position of the subscriber. Accordingly, a number of systems have been proposed to assist in the location determination and provide more accurate position information. One system utilizes a triangulation or arcuation process to determine an approximate location of the caller through an analysis of signal strength measurements and/or propagation delay times of the cellular communications. Another system utilizes the existing Global Positioning System (GPS) with a GPS receiver attached to the cellular telephone to obtain geo-coordinates for the mobile station.
Although many systems have been proposed for more precisely identifying the location of a mobile station, it is equally important that the determined position information, regardless of its accuracy, be provided and distributed through the network when needed. Consider, for example, the situation where a law enforcement agency desires to know the location of either a mobile station or the person using the mobile station. Thus, for a mobile station located in a stolen vehicle, the law enforcement agency would want to monitor the location of the mobile station, and hence the stolen vehicle itself, to assist in apprehending the thief. Alternatively, a law enforcement agency may have an interest in monitoring not only the cellular telephone communications made by a cellular service subscripting suspect, but also the locations from which those calls are occurring. It is necessary then that the law enforcement agency be able to instruct the cellular telephone network to make a position determination on a particular mobile station and relay that information back for use in tracking the suspect.
Furthermore, cellular subscribers now frequently use their mobile stations to make emergency (911) calls. Unlike conventional land line telephones, mobile stations have no fixed address relating to a location which may be obtained by the public safety answering point (PSAP) when an emergency call is made. It is similarly necessary then that the public safety answering point be able to instruct the cellular telephone network to provide updated position information on a particular mobile station from which an emergency call originates and relay that information back for use in dispatching emergency service aid.
In yet another example, with the implementation of an Intelligent Network (IN) architecture, a number of new service features are being provided which require knowledge of mobile station position. For instance, with location dependent call forwarding, the C-number to which a call is transferred depends on where in the network the called mobile station is located. Location dependent network handling is further becoming more of an issue with respect to the service actions taken by other network nodes such as a home location register (HLR). It is likewise necessary then that a network node, such as a service control point in an Intelligent Network or a home location register, be able to signal the cellular telephone network to provide updated position information on a particular mobile station and relay that information back for use in triggering or completing service provision.
In conventional call handling situations, obtaining location information does not present a significant problem as the requesting entity (law enforcement, PSAP, or network node) may make a request of the mobile switching center serving a cellular call for the location information. The process of obtaining location information is significantly complicated, however, when an inter-exchange hand-off of the cellular call occurs. Requests in this case cannot be directed to the newly serving mobile switching center. The reason for this is that the originally serving (i.e., anchor) mobile switching center handling the call remains in the call connection after hand-off occurs, and knowledge of the currently serving mobile switching center is not readily available. There is a need then for a mechanism which would allow the serving mobile switching center to provide location information to the anchor mobile switching center following an inter-exchange hand-off.